(1) Field of the Invention
The present invention relates to a solid-state imaging apparatus, and particularly to a contact-type solid-state imaging apparatus and the like.
(2) Description of the Related Art
A solid-state imaging apparatus (image sensor) is an essential element of an image input apparatus used for various types of image areas including a digital video camera (DVC), a digital still camera (DSC) and the like. In recent years, along with a spread of paperless information, that is, computerized image and character database, an image scanner apparatus has been increasingly in demand. Thus, it has been greatly demanded for a contact-type solid-state imaging apparatus which is used for the image scanner apparatus to implement downsizing and to realize high resolution.
FIG. 4 is a diagram showing a system structure of a conventional contact-type solid-state imaging apparatus. A light emitting diode 109 irradiates an object (original copy) 103, and a diffused reflection light (incident light) 102 from the object 103 is collected by a refractive index distribution rod lens 112, so as to form an image on a light-receiving device 107. The light-receiving device 107 is formed on a semiconductor integrated circuit 108, and converts an outgoing light 106 having information of contrasting density of the object 103 (that is, strong/weak of light intensity) into an electric signal. The system structure is a general structure of the contact-type solid-state imaging apparatus using a lens array (for example, refer to Patent Reference 1).
FIG. 3A is a diagram showing a structure of the conventional refractive index distribution rod lens 112, while FIG. 3B is a diagram showing a refractive index distribution of the aforesaid refractive index distribution rod lens 112. The refractive index distribution rod lens 112 is a cylindrical transparent lens in which a refractive index is changed continuously from the center toward the edge, and has a two-dimensional refractive index distribution in which the refractive index is decreased when a radius is increased. FIG. 2 is a diagram showing a transmission of a light in the refractive index distribution rod lens 112. A light incident from the edge of the lens passes through the lens curving a sine wave, and goes out from the other edge of the lens. At this time, the light convergence can be controlled depending on the lens length. An inverted image at the same magnification is obtained when the lens length is equal to ¼ of a sine wave cycle, an inverted image on the edge side is obtained when the lens length is equal to ½ of a sine wave cycle, an erected image at the same magnification is obtained when the lens length is equal to ¾ of a sine wave cycle, an erected image on the edge side is obtained when the lens length is equal to 1 sine wave cycle. The refractive index distribution rod lens 112 having aforesaid light characteristic can be easily installed in a various types of devices as the cylindrical shape, and the both edge sides of the lens are planes to which the light axis is orthogonal, so that the light axis easily matches the lens axis in an optical system.
However, in the conventional contact-type solid-state imaging apparatus 1000, the lens array is used so as to have an erected image of two-dimensional array. Therefore, it is necessary to have ¾ of a sine wave cycle approximately with high accuracy and have an image which is an identical to the next lens so as to form a rod lens having an accurate refractive index distribution. At present, the refractive index distribution rod lens is formed by two different types of processes mainly. The first process is a monomer volatile process and the other process is inter-diffusion process. In the former process, a monomer on the outer edge is volatilized from a fiber mixture of a low refractive index polymer and a high refractive index monomer, polymerized and cured so as to form a rod lens with refractive index distribution. In the latter process, a monomer group with different refractive index is a concentric fiber structure, and the monomer is diffused between layers, polymerized and cured so as to form a lens. A lens diameter more than 1 mm is necessary in either process, so as to control an accurate refractive index distribution. Thus, a thinning of a rod lens is extremely difficult.
FIG. 1 is a diagram showing the diffused reflection light 102 reflected by the object 103 passes through the refractive index distribution rod lens 112, and is converged at the light-receiving device 107. The refractive index distribution rod lens 112 is an imaging system for imaging at the same magnification, so that the image of the object is projected onto the sensor as the original copy. At this time, the spread of the light is more than 2 mm, which is very large comparing with a pixel size (pixel size is 64 μm at resolution 400 dpi, 42 μm at 600 dpi), and one lens collects the light for plural pixels for imaging. As a result, the signal characteristic is affected by the light-collecting characteristic of the refractive index distribution rod lens 112, so that a shading of contrasting by plural pixel units due to light intensity in the lens occurs.
Patent reference 1: Japanese Laid-Open Patent Application No. H6-342131